The present invention relates to a honeycomb structure and a method for manufacturing the honeycomb structure. More specifically, the present invention relates to a honeycomb structure capable of serving as a honeycomb catalyst body used for purifying target components to be purified contained in exhaust gas discharged from an engine and the like of an automobile by loading a catalyst.
In various kinds of fields such as chemistry, electric power, iron and steel, a ceramic honeycomb structure excellent in thermal resistance and corrosion resistance is employed as a carrier for a catalyst apparatus used for an environmental measure or collection of specific substances or a filter (see, for example, JP-A-2003-33664).
For example, in order to purify exhaust gas discharged from various kinds of engines and the like, there is used a catalyst body (hereinbelow sometimes referred to as a “honeycomb catalyst body”) obtained by loading a catalyst on a honeycomb structure. As shown in FIGS. 4 to 6, such a honeycomb catalyst body 60 using a honeycomb structure 50 is provided with a honeycomb structure 50 having partition walls 52 separating and forming a plurality of cells 54 functioning as fluid passages and a catalyst layer 62 formed by loading a catalyst on the surfaces of the partition walls 52.
As shown in FIGS. 4 and 5, in the case of purifying exhaust gas 68 using such a honeycomb catalyst body 60, the exhaust gas 68 is allowed to flow into the cells 54 from the end face 58 side on one side of the honeycomb catalyst body 60 and brought into contact with the catalyst layer 62 (see FIG. 6) on the surfaces of the partition walls 52 to purify the target components to be purified in the exhaust gas, and then discharged outside from the other end face 59 side.
In the case of purifying exhaust gas using a catalyst body using a honeycomb structure, in order to improve purification efficiency, it is preferable to make a cell hydraulic diameter small and a surface area of the partition wall large to promote transmission of the target components to be purified contained in the exhaust gas from the exhaust gas to the catalyst layer on the surfaces of the partition walls as much as possible.
In order to realize this, there is employed a method where the number of cells per unit area (also referred to as a cell density) is increased. It has been known that the transmission rate of the target components to be purified from the exhaust gas to the catalyst layer on the surfaces of the partition walls increases inversely with the square of cell hydraulic diameter. Therefore, the more the cell density is increased, the more the transmission rate of the target components to be purified is improved. However, since pressure loss also increases inversely with the square of cell hydraulic diameter, there arises a problem of increase in pressure loss in accordance with the rise in transmission rate of the target components to be purified. Therefore, in order to reduce the increase in pressure loss of a honeycomb structure used for a honeycomb catalyst body, various kinds of measures are performed.
In addition, in the case that diffusion speed of the target components to be purified is insufficient in the catalyst layer, purification efficiency tends to decrease in the honeycomb catalyst body. Therefore, in order to enhance purification efficiency for the exhaust gas, it is preferable to reduce the thickness of the catalyst layer, which is generally about tens of micrometers, on the surfaces of the partition walls besides to increase the surface area of the catalyst layer to improve the diffusion speed of the target components to be purified in the catalyst layer. It enables to easily increase the cell density and the surface area of the catalyst layer, thereby improving the transmission rate of the target components to be purified. However, such a method does not solve the problem of the increase in pressure loss.
Further, by increasing the inflow diameter of the honeycomb catalyst body and reducing the flow rate of the exhaust gas to be circulated, the pressure loss can be reduced while maintaining or raising the purification efficiency of the exhaust gas. However, in the case that a honeycomb catalyst body is large, a problem of difficulty in mounting the honeycomb catalyst body on an automobile remains since the space for mounting is limited.
Therefore, in order to reduce the increase of pressure loss of a honeycomb structure used for a honeycomb catalyst body, various kinds of measures are performed (see, for example, JP-A-2002-219319, JP-A-2002-301323, and JP-A-2007-152342).
However, in the methods described in JP-A-2002-219319, JP-A-2002-301323, and JP-A-2007-152342, the increase in pressure loss of the honeycomb structure is not sufficiently reduced, and development of a honeycomb structure having a large average pore size with which the increase in pressure loss can effectively be reduced has been desired.